The invention relates to a process and device for monitoring at least one operating parameter of the core of a nuclear reactor, in particular a pressurized water nuclear reactor.
Nuclear reactors such as pressurized water nuclear reactors have a core made up of juxtaposed fuel assemblies which are generally of straight prismatic shape with their longitudinal axes in the vertical direction, that is along the height of the core.
During operation of the nuclear reactor, it is necessary to make sure that the reactor operates permanently under perfect conditions and in compliance with the general conditions of safety set by regulations and standards.
In particular, it is necessary to determine whether the bulk production and distribution of the neutron flux as well as the bulk distribution of the power released in the core comply with normal and satisfactory operating conditions of the core.
In order to do this, operating parameters of the nuclear reactor core, such as the bulk power distribution in the core, the neutron flux shape factors or even the critical heating ratio, need to be calculated. These parameters are determined from neutron flux measurements in the core which enable the neutron flux distribution throughout the entire core to be determined in three dimensions.
The parameters which are characteristic of the core state under normal operation and which are determined from the neutron flux measurements must not at any time exceed limits determined during design studies of the nuclear reactor.
When a limiting value is exceeded by one of the parameters which are characteristic of core operation, detection of this must raise an alarm and various measures relating to running of the nuclear reactor must be taken.
To monitor the nuclear reactor core operation efficiently, it is necessary to determine the core operating parameters and therefore the neutron flux distribution in the core, in as brief a time as possible.
Neutron flux measurements in the core needed for continuous monitoring of the nuclear reactor in operation are generally carried out by chambers located outside the reactor vessel and are generally designated as xe2x80x9cex-corexe2x80x9d chambers.
These chambers, which have several (for example six) measurement stages over the height of the core, are generally arranged to carry out measurements in four zones on the periphery of the nuclear reactor core and are located symmetrally with respect to two planes of axial symmetry of the core which make an angle of 90xc2x0 between them.
The staged chambers of the ex-core detectors enable flux measurements to be obtained at various levels over the height of the core and in the four zones distributed, around the core, in the circumferential direction. However, these external devices only provide approximate values of the neutron flux inside the core and a rough representation of the neutron flux distribution. As a result, the monitoring parameters obtained lack accuracy and, for safety, greater margins must be provided for the critical values of those parameters which must not be reached or exceeded.
To obtain a more accurate representation of the neutron flux distribution in the core, additional neutron flux measurements are carried out inside the core, at regular but relatively long time intervals, for example of the order of one month, using very small measurement probes, called xe2x80x9cin-corexe2x80x9d probes, which generally consist of fission chambers. The in-core probes are all attached to the end of a cable, called a teleflex cable, which is flexible to ensure that it moves inside a measurement channel of the nuclear reactor instrumentation. Each of the measurement channels comes out at one of its ends into an instrumentation room located at the bottom of the reactor building. The movement of the fission probes in the measurement channels are checked from the instrumentation room. Each measurement channel has, inside the nuclear reactor core, an instrumentation tube of a fuel assembly and a thimble placed inside the instrumentation tube, in which the fission probe moves around. Neutron flux measurements are carried out in a set of fuel assemblies distributed throughout the core cross section.
For example, in a core containing 177 fuel assemblies, there are generally 56 measurement channels. Similarly, there are 58 measurement channels for a core of 193 fuel assemblies, 50 measurement channels for a core of 157 fuel assemblies and 60 measurement channels for a core of 205 fuel assemblies. The neutron flux measurements are carried out while the in-core probes are slowly moved over the entire height of the core. Thus, neutron flux can be measured. at several points, over the height of the core, with a small spacing. Furthermore, given the distribution of instrumented fuel assemblies within the core and the core symmetries, an image which is sufficiently representative of the neutron flux is obtained in the form of a flux map. However, the in-core probes consisting of fission chambers cannot be used for extended periods inside the nuclear reactor core. The accurate determination of the core flux map can only be carried out periodically and therefore cannot be used for continuously monitoring the operation of the nuclear reactor core.
On the other hand, neutron flux measuring probes, which can be positioned and maintained inside the core of a nuclear reactor permanently while the nuclear reactor is in operation, are known. Such neutron flux measuring probes which can be made in the form of xe2x80x9cself-powered neutron detectorsxe2x80x9d, are generally assembled in the form of measuring rods in a vertically aligned arrangement with a constant spacing between two successive probes, to make up the flux measuring detectors over-the entire height of the nuclear reactor core. Each of these rods is introduced into a thimble usually allocated to measurements by mobile probe, which is itself inserted into the instrumentation tube of a fuel assembly. Each of the flux measurement detectors or measuring rods whose length is more or less equal to the height of the core may have, for example, eight measuring probes consisting of self-powered neutron detectors.
It has been proposed, for a nuclear reactor core containing 177 fuel assemblies, to arrange 52 detectors or measuring rods in 52 instrumented assemblies of the nuclear reactor core distributed over the cross section of the core.
Such an instrumentation system, which has 8xc3x9752 measurement points distributed throughout the core, is capable of providing an image of the flux distribution in the. nuclear reactor core in 3 dimensions, with high accuracy.
However the processing of neutron measurements which is carried out by the instrumentation kept permanently in the core during the nuclear reactor operation requires an execution time which can be long, in comparison will the response time needed for monitoring the nuclear reactor core, which makes its use implausible for monitoring core operating parameters.
A process which enables the bulk flux distribution and, from this distribution, the reactor core monitoring parameters to be obtained, both accurately and quickly, is not known.
Generally, nuclear power stations have several power units, each unit consisting of a nuclear reactor located in a reactor building and a conventional part for electricity generation. In this case, the monitoring relates to the nuclear reactors of each power unit.
The purpose of the invention is therefore to propose a monitoring process for at least one operating parameter of the core of a nuclear reactor of a power unit in a nuclear power station, made up of a number of juxtaposed fuel assemblies arranged over the height of the core, by using a set of detectors for measuring neutron flux introduced into at least some of the fuel assemblies of the core, each set comprising a number of probes for measuring neutron flux which are distributed over the height of the core, this process enabling a fast and accurate determination of the flux distribution in the core and of at least one operating parameter of the core to be obtained from this distribution.
For this purpose, during operation of the nuclear reactor, at specified time intervals:
the neutron flux bulk distribution is measured, using the set of neutron flux detectors, the maximum number n of detectors being equal to 15% of the number of fuel assemblies in the core,
a neutron flux calculation code is used and measurements provided by the set of neutron flux detectors are taken into account to obtain the instantaneous neutron flux bulk distribution throughout the entire core, in the form of a set of neutron flux values at points distributed throughout the core,
at least one core operating parameter is calculated from the instantaneous neutron flux bulk distribution and,
an alarm is raised if at least one operating parameter is outside the set range.
According to a first embodiment of the invention, to obtain the instantaneous distribution of the neutron flux throughout the entire core:
an instantaneous calculation is carried out, on the reactor site, of the bulk flux distribution inside the core, at the various points distributed throughout the core comprising a first sub-set of instrumented positions where the neutron flux measuring probes are located and a second sub-set of non-instrumented positions, from parameters originating from the power unit plant, using the neutron flux calculation code,
the differences between the flux values obtained by measurement and the corresponding values calculated from the parameters originating from the power unit plant are calculated for each instrumented position,
the corresponding differences for every non-instrumented position in the core are calculated from the differences relating to the instrumented positions, by an extrapolation procedure,
the values of the said differences are added algebraically to the bulk flux distribution values obtained from the parameters originating from the power unit plant, to obtain the measured value of the bulk flux distribution for every point distributed throughout the core.
According to a second embodiment of the invention, to obtain the instantaneous neutron flux distribution throughout the core:
a first instantaneous calculation is carried out, on the reactor site, of the neutron flux bulk distribution inside the core in the form of a set of neutron flux values at the various points distributed throughout the core, comprising a set of instrumented positions in which the neutron flux measurement probes are located, from parameters originating from the power unit plant, using the neutron flux calculation code,
the differences between the neutron flux values obtained by measurement and the corresponding values calculated from the parameters originating from the power unit plant are calculated for every instrumented position,
the calculated differences are used to correct the defining parameters of the neutron flux calculation code, and
a second instantaneous calculation is carried out, on the nuclear reactor site, of the instantaneous neutron flux distribution inside the core, from the parameters originating from the power unit plant, using the neutron flux calculation code which includes corrected defining parameters.
In order to make the invention clearly understood, the implementation of the process according to the invention for the monitoring of a pressurized water nuclear reactor core and a device enabling this monitoring to be carried out will now be described, with reference to the appended figures.